Depletion of hematopoietic stem cell reserves, expressed as the shortening of leukocyte telomere length (LTL), sets a limit on longevity and increases the risk of cardiovascular disease. Until now, almost all research on LTL has focused on adults. Recent evidence suggests, however, that the factors which influence LTL from conception to birth could be as important as those which influence LTL over the remainder of the life course. The inter-individual variation in LTL at birth is so large that it rivals the averge shortening of LTL over the entire adult life course. In addition, higher paternal age at conception is associated with longer LTL in adult offspring, and since this effect must derive from the paternal germ line, it may be evident by the time of birth. However, no rigorous study has been done of the determinants of LTL at birth. This prospective investigation of the determinants of LTL at birth is an ancillary study which builds on a multisite NICHD-funded parent study of nulliparous women. The ancillary study will be done in 1,000 trios of mother, father, and baby. It will add to the rich array of data in the parent study in several ways, including the collection of paternal blood samples and the measurement of LTL in mothers, fathers, and babies for the 1,000 trios. The Aims of the study are to: 1) examine the relation of paternal age at conception (PAC) to LTL at birth, 2) examine whether maternal exposures and conditions are determinants of LTL at birth, 3) compare LTL at birth across sex, race/ethnicity and socioeconomic status, and 4) explore the relation of fetal growth rate to LTL at birth. This study will build a platform or further investigations including genetic and epigenetic influences on LTL at birth and changes in LTL during early childhood years. By identifying the determinants of LTL at birth, this research will provide a foundation for linking experience from conception to birth with health and longevity in later life. Moreover, this research has the potential to transform understanding of population health by opening novel investigations of the pathways through which intra-uterine experiences are biologically embedded in the individual's constitution, and might be reflected in risk factors for disease which emerge in childhood and evolve thereafter.